The present invention relates generally to the fields of microscopy and metrology, and more specifically to interference microscopes used for non-contact, ultra-high resolution optical profiling and metrology of integrated circuits and MEMS devices.
Microelectromechanical systems (MEMS) devices are used for a variety of applications including optical switches and displays, microrelays, accelerometers, gyroscopes, image correctors, ink jet printheads, flow sensors, and medical devices. MEMS are fabricated in a fashion similar to microelectronics in the integrated circuit (IC) industry using surface micromachining techniques. Freestanding MEMS structures, such as pivoting mirrors, beamsplitters, lenses, gears, cantilevered beams, and motors, etc. are created at the end of the process flow by removing the oxide matrix surrounding thin film structural members. Polycrystalline silicon (i.e., polysilicon) is to date the most successful MEMS material because many requirements can be satisfied simultaneously. Other structural materials are in use or being explored, such as: aluminum, silicon carbide and xe2x80x9camorphous diamondxe2x80x9d.
Surface micromachining, LIGA techniques, and thin film techniques such as chemical vapor deposition, sputtering, or pulsed laser ablation can be used to form MEMS structures. For volume production, the same MEMS device will be repeatedly fabricated over the surface of a large diameter (4-12 inches) silicon wafer. Typically, there are fifty or more identical die sites. The microstructure of the resulting films and structures can exhibit cross-wafer non-uniformities, resulting in variations of thickness, height, residual stress, stress gradient, or elastic modulus across the wafer. Both mechanical and surface properties must be sufficiently well controlled to guarantee that the intended design function of the MEMS device is met across the entire wafer. For example, the resonant frequency of an electrostatic comb drive can be sensitive to small variations in residual stress. Also, highly curved comb drive fingers or suspensions (caused by stress gradient) will result in device malfunction. Furthermore, surface properties such as adhesion and friction are very sensitive to processing, and may exhibit cross-wafer non-uniformity as well. Poor quality control of surface properties may result in failure of devices that rely on contact or sliding of surfaces.
A need exists, therefore, for rapid and accurate, non-contact, three-dimensional imaging and metrology of complex features of MEMS structures (as well as other structures, such as thin films, microfluidic channels, and biological specimens). One conventional metrology technique is SEM. However, because of charging and calibration problems, it is difficult to obtain the required nanometer scale resolution by this technique. Other metrology techniques, such as AFM and contact profilometry, can provide the required nanometer-scale resolution to accurately measure 3-D out-of-plane features of IC""s and MEMS devices, but either require extensive sample preparation, or rely on potentially destructive contact with the sample surface. Other non-contact techniques, such as conventional light microscopy, do not provide the required resolution.
In U.S. Pat. No. 5,990,473, Dickey and Holswade describe an apparatus and method for sensing motion of MEMS structures by reflecting or scattering light off of a corrugated surface (e.g., gear teeth) of a movable member (e.g., a gear). However, this system does not provide nanometer-scale measurement of the surface topography of the MEMS structures.
Optical interference microscopes (e.g., optical profilers) can provide the required accuracy (nanometers to sub-nanometers). These non-contact, non-destructive devices use quantitative interferometric phase-measurement techniques to determine the phase difference between an object point on the sample and a reference surface (typically an optically flat reference mirror). The measured phase difference is then converted into topological information. Computerized analysis of a series of interferograms taken while the reference phase of the interferometer is changed (e.g., by using phase-shifting interferometry) provides rapid and accurate determination of the wavefront phase encoded in the variations of the intensity patterns of the recorded interferograms, requiring only a simple point-by-point calculation to recover the phase. The use of phase-shifting interferometry (PSI) conveniently eliminates the need for finding and following fringe centers. PSI is also less sensitive to spatial variations in intensity, detector sensitivity, and fixed pattern noise. Using calibrated PSI, or similar computer analysis techniques, measurement accuracies as good as 0.1 nanometers can be attained if there are no spurious reflections from interfaces other than the one of interest.
It is highly desirable to perform metrology of IC""s and MEMS devices at the wafer scale using a microscope setup on a conventional microelectronics probe station that can align wafers and move rapidly from one die site to the next. During electrical probing of a wafer on the probe station, released MEMS structures can be electrically activated; hence, their motion or mechanical behavior can be tested at the wafer scale (e.g., before the wafer is sliced into individual dies). Consequently, a need also exists for measuring out-of-plane deflections, oscillations, or other dynamic 3-D parameters of actuated MEMS devices with high accuracy and low cost. Electrical probing of the wafer requires a long working distance between the end of the microscope (e.g., tip of the sample objective) and the face of the wafer to permit access from the side of the wafer by a standard commercial electrical probe arm or probe card. The required working distance can be as large as 20-30 mm, depending on the number and size of probes needed to simultaneously reach across the wafer from the side.
Commercially available interference microscopes (e.g., the New View 5000 3-D Surface Profiler manufactured by Zygo, Inc., Middlefield, Conn., or the NT2000 3D Optical Profiler manufactured by Wyko, Inc. of Tuscan, Ariz.) do not have the necessary long working distance required for imaging MEMS structures while being actively probed. Typically, commercial interference microscopes have a free working distance less than approximately 10 mm. This is because they use a special interferometer attachment (e.g., Mirau, Fizeau, or Michelson interference attachment), which contains a beamsplitter and reference mirror surface in a compact arrangement. The interferometer attachment is commonly located in-between the standard sample objective and the sample""s surface. This arrangement unfortunately reduces the available free working distance to less than 10 mm (especially at higher magnifications, e.g., 20-50xc3x97). Additionally, in this configuration interference fringes cannot be easily obtained through a transparent window (such as might be found in a vacuum chamber) due to the phase shift induced by the window. A need exists, therefore, for an interferometric microscope that has a long working distance, and that can easily image through a transparent window.
Historically, the Linnik interference microscope (i.e., microinterferometer) has been used to provide a long working distance, including high magnification objectives having high numerical apertures. See U.S. Pat. No. 4,869,593 to Biegen; also U.S. Pat. No. 5,956,141 to Hayashi; also Advanced Light Microscoy, Vol. 3, by Maksymilian Pluta, Elsevier Science Publishers, Amsterdam, 1993, pp. 334-347.
FIG. 1 illustrates a schematic layout of a standard Linnik microinterferometer, which is based on a Michelson-type two-beam interferometer, and uses a pair of well-matched sample objectives. The illumination beam is split into two beams by means of a beamsplitter. The reference beam in the reference arm is directed onto and reflects off of a reference surface (i.e., the reference mirror). The object beam (i.e., sample beam) in the sample arm (i.e., sample circuit) impinges onto and then reflects off of the sample""s surface (e.g., MEMS device). The two beams are then recombined after passing back through the beamsplitter, thereby forming an interferometric image (i.e., interferogram) of the sample""s surface at the image plane of the microscope.
Most commercially available interference microscopes utilize an incoherent source of light, which limits the coherence length to approximately. 50 xcexcm. With such a short coherence length, the optical path lengths of the reference arm and the object/sample arm must not differ by more than approximately 5 xcexcm in order to achieve high contrast interference fringes. An additional requirement is that straight interference fringes be obtained when viewing a sample having an optically flat surface. This requirement is only satisfied when the divergence of the reference beam precisely matches that of the sample beam. When using incoherent light, these two requirements imply that the focal lengths and optical path lengths of the first objective in the sample arm and the second objective in the reference arm must be precisely matched. These requirements are satisfied in a standard Linnik interferometric microscope by optically testing a batch of sample objectives and selecting well-matched pairs of objectives. As illustrated in FIG. 1, a pair of well-matched objectives is used to produce high-contrast interference fringes with minimum curvature. It is difficult and, hence, expensive to obtain two closely matched long working distance (LWD) sample objectives, especially at high magnification (e.g., 50X). In addition, changing the overall magnification of the microscope (e.g., from 5X to 20X) requires that two objectives be changed, which is time-consuming and expensive. However, the requirement for equal divergence (i.e., wavefront curvature) of the two beams remains.
The use of laser illumination in a Linnik interference microscope can alleviate the problems associated with mismatched optical path lengths because of the long coherence lengths characteristic of laser light. Using laser light, high quality interferograms can be obtained, even when the reference and the sample arms have substantially different optical path lengths.
Gale, et al. describe a Linnik microscope capable of illumination by one of two different""sources, namely, a tungsten halogen lamp or a helium-neon laser (see D. M. Gale, M. I . Pether, and J. C. Dainty, xe2x80x9cLinnik Microscope Imaging of Integrated Circuit Structuresxe2x80x9d, Applied Optics Vol. 35, No. 1, January, 1996, pp. 131-137). However, this system uses a pair of well-matched sample objectives (with an aberration less than lambda/8), because this is required when the incoherent tungsten halogen lamp is used (i.e., due to the requirement for matching both the optical path lengths and beam divergence when using incoherent light).
The need remains, therefore, for a long working distance interference microscope that does not require the use of an expensive, well-matched pair of sample objectives. A need exists, also, for an interference microscope that uses an inexpensive reference beam circuit that requires only minor adjustments when changing magnifications. There is also a need for allowing imaging through a glass window, requiring only minor adjustments of the reference beam circuit. Against this background, the present invention was developed.